The present invention generally relates to masks, mask producing methods and pattern forming methods using masks, and more particularly to a mask which uses a phase shift of light, a mask producing method for producing such a mask and a pattern forming method which uses such a mask.
When forming patterns of elements, circuits and the like on a semiconductor wafer in a production process of a semiconductor device, it is normal to employ a pattern transfer exposure which uses an ultraviolet light.
The pattern which is to be transferred onto the wafer is formed depending on the existence of a metal thin film which is provided on a glass substrate, where the metal thin film is opaque with respect to the light and the glass substrate is transparent with respect to the light. The pattern on the glass substrate to be transferred onto the wafer is called a mask when this pattern is identical to a chip pattern which is actually transferred onto the wafer. On the other hand, the pattern on the glass substrate is called an enlarged mask or a reticle when this pattern is enlarged compared to the chip pattern which is actually transferred onto the wafer. When using the mask, the pattern is transferred onto a resist layer on the wafer and exposed using a parallel ray. On the other hand, when using the reticle, the pattern is transferred onto the resist layer on the wafer and exposed using a reduction lens system which projects a reduced pattern on the resist layer.
In order to improve the resolution of the pattern which is transferred especially when the integration density of the integrated circuit is large and the pattern is fine, it is necessary to improve the contrast at an edge portion of the region which is exposed.
FIG. 1 shows an example of a conventional mask which is made up of an opaque layer having a predetermined pattern and a transparent substrate. FIG. 2 shows an example of an optical system for forming a pattern on the wafer using the mask shown in FIG. 1.
In FIG. 1, a mask 450 comprises a transparent substrate 452 and an opaque layer 451 which is made of a material such as chromium (Cr). The opaque layer 451 is formed to a predetermined pattern by lithography and etching processes.
In FIG. 2, a light C which is emitted by an exposure apparatus (not shown) illuminates the mask 450. The light C cannot be transmitted through the opaque layer 451 but is transmitted through a portion of the transparent substrate 452 not provided with the opaque layer 451. The transmitted light passes through an imaging lens system 453 and exposes a resist material 455 which is coated on a wafer 454. For example, a resist OFPR manufactured by Tokyo Ooka Kogyo K. K. of Japan may be used as the resist material 455. As a result, a pattern identical to the pattern of the mask 450 is formed on the wafer 454 by an etching process.
When forming the pattern using an optical lens system, the exposure on the wafer 454 is made based solely on data related to the contrast which is determined by the existence of the opaque layer 451 on the transparent substrate 452. For this reason, there is a physical resolution limit to the pattern formation due to the wavelength of the light which is obtained via the optical lens system, and it is difficult to form a fine pattern depending on the wavelength of the light used for the exposure.
Conventionally, the photolithography process uses a reticle which is made by forming an opaque layer on a transparent substrate and patterning the opaque layer. For example, the opaque layer is made of Cr and the transparent substrate is made of a transparent material such as glass and quartz. FIGS. 3A through 3D are diagrams for explaining such a conventional pattern forming method.
In FIG. 3A, the optical system includes a light source 461, an illumination lens 462, and an imaging lens system 465. A reticle 464 is arranged between the illumination lens 462 and the imaging lens system 465. The light source 461 is made of a mercury lamp, an excimer laser or the like and is provided with a filter for emitting i-ray or g-ray. The light 463 from the light source 461 illuminates the reticle 464 via the illumination lens 462. For example, the partial coherency .sigma. of the illumination lens 462 is 0.5. The reticle comprises a transparent substrate 468 and an opaque pattern 469 which is formed on the transparent substrate 468. For example, the transparent substrate 468 is made of glass and the opaque pattern 469 is made of Cr. The opaque pattern 469 on the reticle 464 is imaged on a photoresist layer 467 which is formed on a semiconductor substrate 466 via the imaging lens system 465. For example, the imaging lens system 465 has a numerical aperture (NA) of 0.50.
According to this pattern forming method, the resolution can be described by K1.multidot..lambda./NA, where K1 denotes a process coefficient which is normally 0.6 to 0.8 and .lambda. denotes the wavelength of the light. The wavelength .lambda. of the light 463 is approximately 365 nm in the case of the i-ray which is emitted using the mercury lamp and is 248 nm or 198 nm when the excimer laser is used. The numerical aperture NA differs depending on the imaging lens system 465 which is used but is approximately 0.5, for example. In order to improve the resolution, it is necessary to set K1 or .lambda. to a small value and set NA to a large value. However, the values of K1 and NA cannot be set freely. In addition, the value of .lambda. is restricted by the light source 461 and the optical system used. The resolution is determined when the wavelength .lambda. of the light used for the exposure, the numerical aperture NA and the process coefficient K1 are determined, and it is impossible to image a pattern finer than the resolution.
The light 463 emitted from the light source 461 illuminates the entire surface of the reticle 464, and the portion of the light 463 which illuminates the opaque pattern 469 is stopped by the opaque pattern 469. For this reason, only the portion of the light 463 which illuminates the portion of the reticle 464 not provided with the opaque pattern 469 is transmitted through the reticle 464 and is imaged on the photoresist layer 467 via the imaging lens system 465. FIG. 3B shows an electrical vector E of the light transmitted through the reticle 464, and FIG. 3C shows a light intensity P of the light transmitted through the reticle 464. A pattern having a light intensity distribution proportional to the square of the amplitude of the light illuminating the photoresist layer 467 is formed on the photoresist layer 467 and the photoresist layer 467 is selectively exposed.
FIG. 3D shows a portion of the reticle (opaque mask) 464 on an enlarged scale. A minimum width W of the pattern which can be exposed is restricted by the resolution which is determined by the imaging lens system 465.
Next, a description will be given of the light intensity distribution for cases where the pattern which is exposed is a line pattern which is finer than the resolution, by referring to FIGS. 4A through 4D. In FIGS. 4A through 4D, the wavelength .lambda. of the light used is 365 nm, the numerical aperture NA is 0.50 and the partial coherency .sigma. of the light is approximately 0.5.
FIG. 4A shows the light intensity distribution for the case where the line pattern which is imaged has a width of 0.35 .mu.m. The light intensity approaches zero approximately at a center position (0.0) and gradually rises on both sides of the center position (0.0). The line width is approximately 1.0 .mu.m or greater at a position where the light intensity is a maximum. When the photoresist layer is developed using a position where the light intensity is approximately 0.2 as a developing threshold, it is possible to develop a pattern which has a designed width in the order of 0.35 .mu.m.
FIG. 4B shows the light intensity distribution for the case where the line pattern which is imaged has a width of 0.30 .mu.m. A minimum of the light intensity at the center position (0.0) is increased compared to the light intensity distribution shown in FIG. 4A. The width of the light intensity distribution itself shown in FIG. 4B is not much different from that of FIG. 4A.
FIGS. 4C and 4D respectively show the light intensity distributions for the cases where the line pattern which are imaged have widths of 0.25 .mu.m and 0.20 .mu.m. In each of FIGS. 4C and 4D, the minimum of the light intensity at the center position (0.0) is increased compared to the light intensity distribution shown in FIG. 4A, similarly to the case shown in FIG. 4B. The width of the light intensity distribution itself shown in each of FIGS. 4C and 4D is not much different from that of FIG. 4A. In other words, even when the pattern width is reduced exceeding the resolution, the pattern width of the light intensity distribution which is obtained does not decrease and the minimum of the light intensity at the center position (0.0) increases. In these cases, it is impossible to reduce the line width which is exposed, and the black level is exposed as a gray level. For these reasons, it is impossible to form an image which is finer than the resolution.
On the other hand, a method of shifting the phase of the light which is transmitted through the mask and exposed on the wafer by 180.degree. depending on the patterns of the mask is proposed in Marc D. Levenson, "Improving Resolution in Photolithography with a Phase-Shifting Mask", IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-29, No. 12, December 1982. According to this proposed method, the interference between the patterns is eliminated so as to improve the contrast on the wafer and improve the resolution of the exposure apparatus.
However, it is difficult to apply this proposed method to masks and reticles having fine patterns, and there is a problem in that it is troublesome to generate pattern data peculiar to the phase shift pattern. For this reason, there is a demand to realize a phase shift pattern which is easily applicable to masks and reticles having fine patterns and does not substantially increase the number of processes such as the generation of the pattern data.
The phase shift pattern of the phase-shifting mask which is used according to this proposed method is formed as follows. First, an auxiliary pattern is formed in a vicinity of a design pattern (white pattern) which is to be transferred onto the wafer, where the auxiliary pattern has a width smaller than that of the design pattern. Second, a phase shifter is formed on the auxiliary pattern. The phase shifter is an organic pattern made of a resist or the like which is formed by coating, exposure and developing processes, or an inorganic pattern which is formed by chemical vapor deposition and lithography processes. For example, a description will now be given of a method of forming a phase-shifting mask which uses a negative resist pattern as the phase shifter, by referring to FIGS. 5A through 5D.
In FIG. 5A, an opaque layer 552 is formed on a glass substrate 551. An aperture pattern, that is, a design pattern 553 comprising a transmission region, and fine patterns 554A and 554B are formed in the opaque layer 552 by a lithography using an electron beam exposure. For example, the design pattern 553 has a width in the order of 1.5 .mu.m, and the fine aperture patterns 554a and 554B have a width in the order of 0.5 .mu.m. The fine aperture patterns 554A and 554B are formed as auxiliary patterns in a region neigboring the design pattern and separated by a distance in the order of 0.5 .mu.m, for example.
Next, as shown in FIG. 5B, a transparent conductor layer 555 for preventing charge up at the time of the electron beam exposure is formed on the surface of the glass substrate 551 including the inner sides of the aperture patterns 553, 554A and 554B.
Then, as shown in FIG. 5C, a negative resist layer 656 is formed on the glass substrate 551 to a thickness such that the phase of the light which is transmitted through the negative resist layer 656 is shifted by 180.degree.. A prebaking process is made if needed, and a phase shift pattern is formed on the fine patterns 554A and 554B by the electron beam exposure.
A thickness D of the negative resist layer 656 can be obtained from the following formula (1), where .lambda. denotes the wavelength of the light used for the exposure and n denotes a refractive index of the shifter material. EQU D=.lambda./2(n-1) (1)
When using the i-ray having the wavelength of 365 nm for the exposure, the refractive index n of the negative resist layer 656 is approximately 1.6 and the thickness D becomes approximately 304 .mu.m.
Next, as shown in FIG. 5D, a developing process is made to selectively form phase shift patterns 556A and 556B on the fine patterns 554A and 554B, respectively. The phase shift patterns 556A and 556B is made up of the negative resist layer 656 which has the thickness D.
FIG. 6 shows a phase profile corresponding to the pattern position of the i-ray transmitted through the phase-shifting mask shown in FIG. 5D when the i-ray is used for the exposure.
On the other hand, when a positive resist is used, a phase shift pattern 557 which is made up of the positive resist is selectively formed on the design pattern 535 as shown in FIG. 7 by processes similar to those described in conjunction with FIGS. 5A through 5D. In FIG. 7, those parts which are essentially the same as those corresponding parts in FIGS. 5A through 5D are designated by the same reference numerals, and a description thereof will be omitted.
FIG. 8 shows a phase profile corresponding to the pattern position of the i-ray transmitted through the phase-shifting mask shown in FIG. 7 when the i-ray is used for the exposure.
According to the phase-shifting mask shown in FIGS. 5D and 7, the phase of the i-ray (i.sub.a and i.sub.c) which is transmitted through the design pattern 553 and the phase of the i-ray (i.sub.b and i.sub.d) which is transmitted through the fine patterns (auxiliary patterns) 554A and 554B differ by 180.degree. as may be seen from FIGS. 6 and 8. For this reason, the i-ray (i.sub.a and i.sub.c) which is scattered in the horizontal direction from the region immediately below the design pattern is cancelled by the i-ray (i.sub.b and i.sub.d) which is scattered in the horizontal direction from the region immediately below the auxiliary pattern, and the contrast at the ends of the exposure region is improved thereby improving the resolution.
The aperture width of the auxiliary pattern is made narrow to such an extent that the light used for the standard exposure is insufficient to expose the resist layer to the bottom portion thereof. Hence, the auxiliary pattern will not be transferred onto the wafer when the exposure is made using the phase-shifting mask.
For example, the methods described in conjunction with FIGS. 5 through 8 are proposed in Japanese Laid-Open Patent Applications No. 61-292643, No. 62-67514 and No. 62-18946.
However, the conventional masks suffer from the following disadvantages.
First, in the case of the mask which uses no phase shift, it is difficult to form a pattern which is narrower than the wavelength of the light due to the physical resolution limit of the optical system. When an attempt is made to realize a narrow line width, it is necessary to make a structural modification such as reducing the wavelength of the light and increasing the numerical aperture. Accordingly, it is impossible to form the fine patterns which are required in the future integrated circuits using the optical method.
Second, in the case of the mask which uses the phase shift, the pattern formation can only be applied to a pattern such as the so-called line-and-space pattern which has regularity, and the pattern formation cannot be applied to the production of integrated circuits having various patterns. In addition, it is impossible to form a fine pattern because an opaque layer must always be provided.
Third, in the case of the mask which uses the phase shift, the auxiliary pattern must be patterned by the exposure technique to a degree finer that the design pattern. For this reason, the fineness of the design pattern must be restricted in order that the resolution limit of the auxiliary pattern is not exceeded.
Fourth, in the case of the mask which uses the phase shift, the number of steps required to generate the pattern data is large because it is necessary to generate in addition to the data related to the design pattern the pattern data related to the auxiliary pattern, the pattern data related to the phase shift and the like.
Fifth, in the case of the mask which uses the phase shift, it is difficult to control the quality and thickness of the phase shifter which affects the refractive index when an organic material such as a resist is used for the phase shifter. For this reason, it is difficult to form a uniform phase shift pattern which has an accurate phase shift quantity.
Sixth, in the case of the mask which uses the phase shift, the phase shifter is made of a material different from the glass substrate. Thus, a reflection occurs at the boundary of the phase shifter and the glass substrate thereby deteriorating the exposure efficiency.